1. Field of the Invention
This invention relates to a communication system and more particularly relates to determining the switch hook status of terminal equipment of the communication system.
2. Description of the Related Art
Communication systems utilizing transmission lines such as subscriber loops are commonplace throughout much of the world. Subscriber loops are terminated on one end by terminal equipment and provide a path for the terminal equipment to communicate with other terminal equipment via, for example, a vast network of central offices, private branch exchanges, satellite relay systems, transmission lines, repeaters, and wireless systems. FIG. 1 illustrates a portion of a conventional subscriber loop communication system 100. The subscriber loop 102 is modeled as a balanced two-wire transmission line 103, with loop resistances R and inductances L and leakage impedance modeled by capacitor 104 and resistor 106. The subscriber loop 102 is terminated on respective ends by terminal equipment 110 and a central office 122 line card 120. The leakage resistor 106 is generally at least tens of thousands of ohms. The subscriber loop 102 provides a communication path for information transmission such as voice signals and signaling information between a subscriber's terminal equipment and the central office 108.
Terminal equipment 110 is illustratively modeled as a telephone with off- hook resistance 112, nominally 200 ohms, and ringer impedance Z, which may be modeled, for example, as a series RC or series RLC circuit. Terminal equipment 110 includes a switch hook 118 which loads the subscriber loop 102 on the subscriber end with resistance 112 when the terminal equipment 110 is off-hook (as shown) and loads subscriber loop 102 with ringer impedance Z when the terminal equipment 110 is on-hook. Terminal equipment 110 may be any of a variety of devices besides the familiar, ubiquitous telephone such as facsimile machines, private branch exchanges, voice mail systems key telephone systems, computers, modems, telephone answering machines, alarm systems, and radio control systems, as well as many other devices.
The other end of the subscriber loop 102, opposite terminal equipment 110, converges on line card 120 of central office 122. The line card 120 terminates subscriber loop 102 at conductors A (Tip) and B (Ring) with a feed impedance of 900 ohm or other standard feed impedance. The line card 120 provides a gateway to the public switched telephone network (PSTN) through switching network 124.
Referring to FIG. 2, the subscriber line interface circuit (SLIC) 202 of line card 120 provides a two-wire interface 204 to the generally analog signal carrying subscriber loop 102. The SLIC 202 performs a variety of interface functions that allow terminal equipment 110 to communicate with other terminal equipment (not shown). The SLIC 202 and the subscriber loop audio-processing circuit (SLAC) 206 carry out the well-known BORSCHT (Battery feed, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, and Test) functions. The SLIC 202 monitors direct current (DC) levels on the subscriber loop 102 with ground key detector circuitry 208 and off-hook detector circuitry 210. Input decoder and control circuitry 214 provides a mechanism for other circuitry (not shown) in the central office 122 and for SLAC 206 to control such SLIC 202 functions as subscriber loop 102 activation, ringing, and polarity reversal. Analog two-wire interface 204 and signal transmission circuitry 212 cooperate in sensing subscriber loop 102 metallic voltage (voltage at conductor A minus voltage at conductor B or Vab) while generally having a high rejection of longitudinal voltages (Vab_Long). Alternating current (AC) signals, such as voice signals, are transmitted over subscriber loop 102 to terminal equipment 110 by two-wire interface 204 and signal transmission circuitry 212 in response to voice information input signals received from central office 122 through SLAC 206. The ring relay driver 216 activates a relay(s) which connects a ringing signal from a central office 122 ringing AC voltage generator (not shown) and DC voltage bias source, to terminal equipment 110 when a third party is calling. The ring trip detector circuitry 218 detects an off-hook condition of terminal equipment 110 and initiates cessation of the ringing signal application to subscriber loop 102.
The SLAC 206 generally filters and converts analog output signals received from SLIC 202 into digital signals (A/D), processes the signals in accordance with control and timing information, and compresses the digital signals. The pulse code modulation (PCM) interface 220 provides PCM signals to the central office 122. SLAC 206 also generally receives digital audio input signals from the central office 122 via PCM interface 220, expands the digital input signals, processes the signal in accordance with control and timing information, and converts the digital signals into analog signals (D/A) for input to SLIC 202. Additional information on SLICs and SLACs is found in the 1995 Advanced Micro Devices of California data book entitled "Linecard Products for the Public Infrastructure Market."
The power feed controller 222 includes a battery feed circuit and a polarity reversal circuit. The battery feed functions supply direct current from a central office battery (not shown) to the subscriber loop 102 through balanced feed resistances at conductors A and B. Loop current is generally limited to no more than 45 to 75 milliamperes (mA) in a low-resistance subscriber loop. Higher subscriber loop resistances generally result in lower subscriber loop current. The on-hook subscriber loop powering voltage is typically the battery voltages minus 48 Volts DC (Vdc) less any overhead voltage, typically about 4 Vdc, necessary to prevent SLIC 202 saturation. Battery feed specifications are regionally provided and conform with specifications provided by, for example, BELLCORE, the Electronic Industries Association (EIA), British Telecom, and the International Telegraph and Telephone Consultative Committee (CCITT).
Call establishment performance is of considerable importance to telephone companies. Generally, terminal equipment 110 initiates a call by seizing subscriber loop 102 by loop start signaling. Another method, referred to as ground start signaling, is generally used by private branch exchanges (PBX). Loop start signaling occurs after the terminal equipment 110 goes off-hook and switch hook 118 closes the subscriber loop 102 across resistance 112 as shown in FIG. 1. The resulting DC subscriber loop current is detected by line circuit 120, which then connects equipment capable of receiving dialed address information.
Address information may be transmitted by 110 using dual tone multifrequency (DTMF) signal generators (not shown) or with dial pulses from a dial pulse instrument. The pulse rate is typically in the range of 8-12 pulses per second (pps) and may be as high as 20 pps. Pulses are generated by making and breaking (off-hook and on-hook, respectively) the subscriber loop 102 switch hook 118 connection. Generally, the pulse duration is about 100 milliseconds (ms) with the break interval varying from about 55-65 percent of the total pulse duration. The minimum time between dial pulses is typically about 200 ms.
Regional communication authorities and central office 108 service providers generally require a line card to detect transitions between on-hook and off-hook events within a predetermined time, such as 2 ms, after the actual initiation of the transition event. However, meeting this criteria is a non-trivial task due to widely varying subscriber loop 102 characteristic such as impedance.
Subscriber loop 102 off-hook impedance varies widely from as little as about 200 ohms to over 2 kohms. When the terminal equipment 110 goes off-hook, the line impedance of subscriber loop 102 drops suddenly from at least tens of thousands of ohms suddenly to 0.2-2 kohms in a matter of milliseconds. This sudden subscriber loop 102 impedance drop causes the DC current (Idc) in subscriber loop 102 to suddenly increase. The sudden rise in Idc can saturate the circuitry switch hook detection circuitry in the off-hook detector 210. It may be difficult for the off-hook detector 210 to recover from saturation and accurately detect off-hook conditions within the required predetermined time.
Although the analog technology used to implement SLIC 202 may be optimized to operate within a small range of subscriber loop 102 characteristics, the subscriber loop 102 characteristics are dynamic, widely varying, and difficult to reliably accommodate. Also, analog circuitry often suffers from well-known aging side effects such as instability and circuit parameter drifting which may affect long-term reliability. Moreover, analog circuit features in an integrated circuit are large, presently in general on the order of about 7 .mu.m, which increases costs of analog integrated circuitry. Furthermore, providing a cost efficient common hardware platform which lends itself to cost effective modifications to conform with various regional standards and operating environments is at least a difficult problem.